FIG. 17 corresponds to FIG. 1 of Japanese Patent No. 4002969, showing a conventional gas sensor. This gas sensor comprises a thermoelectric conversion element 91 and a catalyst layer 92 arranged thereon on a substrate 93. When gas to be detected is adsorbed on the catalyst layer 92, the catalyst layer 92 generates heat. The thermoelectric conversion element 91 converts the heat into an electric power to detect the gas.